Light rail

Many original tram and streetcar systems in the United Kingdom, United States, and elsewhere were decommissioned starting in the 1950s as the popularity of the car increased. Britain abandoned its tram systems, except for Blackpool, with the closure of Glasgow Corporation Tramways (one of the largest in Europe) in 1962.[9]

Although some traditional trolley or tram systems exist to this day in San Francisco cable car system and streetcars in New Orleans, the term "light rail" has come to mean a different type of rail system. Modern light rail technology has primarily West German origins, since an attempt by Boeing Vertol to introduce a new American light rail vehicle was a technical failure. After World War II, the Germans retained many of their streetcar networks and evolved them into model light rail systems (Stadtbahnen). Except for Hamburg, all large and most medium-sized German cities maintain light rail networks.[10]

The basic concepts of light rail were put forward by H. Dean Quinby in 1962 in an article in Traffic Quarterly called "Major Urban Corridor Facilities: A New Concept". Quinby distinguished this new concept in rail transportation from historic streetcar or tram systems as:

• having the capacity to carry more passengers
• appearing like a train, with more than one car connected together
• having more doors to facilitate full utilization of the space
• faster and quieter in operation

The term light rail transit (LRT) was introduced in North America in 1972 to describe this new concept of rail transportation.[11]

The first of the new light rail systems in North America began operation in 1978 when the Canadian city of Edmonton, Alberta, adopted the German Siemens-Duewag U2 system, followed three years later by CTrain Calgary, Alberta, and San Diego, California. The concept proved popular, and there are now at least 30 light rail systems in the United States and over 40 in North America.

Britain began replacing its run-down local railways with light rail in the 1980s, starting with the Tyne and Wear Metro and followed by the Docklands Light Railway (DLR) in London. The historic term light railway was used because it dated from the British Light Railways Act 1896, although the technology used in the DLR system was at the high end of what Americans considered to be light rail. The trend to light rail in the United Kingdom was firmly established with the success of the Manchester Metrolink system, which opened in 1992.

The most difficult distinction to draw is that between light rail and streetcar or tram systems. There is a significant amount of overlap between the technologies, many of the same vehicles can be used for either, and it is common to classify streetcars or trams as a subcategory of light rail rather than as a distinct type of transportation. The two general versions are:

1. The traditional type, where tracks and trains run along the streets and share space with road traffic. Stops tend to be very frequent, but little effort is made to set up special stations. Because space is shared, the tracks are usually visually unobtrusive.
2. A more modern variation, where the trains tend to run along their own right-of-way, separated from road traffic. Stops are generally less frequent, and the vehicles are often boarded from a platform. Tracks are highly visible, and in some cases significant effort is expended to keep traffic away through the use of special signaling, level crossings with gate arms, or even a complete separation with non-level crossings.

Sydney's Dulwich Hill Line is mostly situated on segregated tracks along a former heavy rail corridor

At the highest degree of separation, it can be difficult to draw the line between light rail and metros. The London Docklands Light Railway would likely not be considered as "light rail" were it not for the contrast between it and the rapid transit London Underground. In Europe and Asia, the term light rail is increasingly used to describe any rapid transit system with a fairly low frequency or short trains compared to heavier mass rapid systems such as the London Underground or Singapore's Mass Rapid Transit. However upon closer inspection, these systems are better classified as light metro or people movers. For instance, Line 1 and Line 3 in Manila are often referred to as "light rail", despite being fully segregated, mostly elevated railways. This phenomenon is quite common in East Asian cities, where elevated metro lines in Shanghai, Wuhan, and Dalian in China; and Jakarta, Greater Jakarta and Palembang in Indonesia are called light rail lines. In North America, such systems are not usually considered light rail.

Hong Kong's MTR Light Rail serves the northwest suburbs with unidirectional high floor LRVs.

Many systems have mixed characteristics. Indeed, with proper engineering, a rail line could run along a street, then go underground, and then run along an elevated viaduct. For example, the Los Angeles Metro Rail's L Line "light rail" has sections that could alternatively be described as a tramway, a light metro, and, in a narrow sense, rapid transit. This is especially common in the United States, where there is not a popularly perceived distinction between these different types of urban rail systems. The development of technology for low-floor and catenary-free trams facilitates construction of such mixed systems with only short and shallow underground sections below critical intersections as the required clearance height can be reduced significantly compared to conventional light rail vehicles.[20]

It is even possible to have high-floor rapid transit cars run along a street, like a tram; this is known as street running.

Belgium's Coast Tram operates over almost 70 km and connects multiple town centres.

In some areas, "light rail" may also refer to any rail line with frequent low speeds or many stops in a short distance. This inherits the old definition of light railway in the UK. Hong Kong's Light Rail is an example of this, although it is also called "light rail" because it is a lower-scale system than the rest of the MTR. Sprinter in the San Diego area uses DMUs and is targeted towards a commuter rail audience; however, because of the large number of stops along the line, it is called light rail.

Reference speed from major light rail systems, including station stop time, is shown below.[21]

However, low top speed is not always a differentiating characteristic between light rail and other systems. For example, the Siemens S70 LRVs used in the Houston METRORail and other North American LRT systems have a top speed of 106 kilometres per hour (66 mph) while the trains on the all-underground Montreal Metro can only reach a top speed of 72 kilometres per hour (45 mph). Los Angeles Metro light rail vehicles have higher top and average speeds than Montreal Metro or New York City Subway trains.[22] The main difference is that Montreal Metro and New York City Subway trains carry far more passengers than any North American LRT system, and the trains have faster acceleration, making station-to-station times relatively short in their densely populated urban areas. Most light rail systems serve less densely populated cities and suburbs where passenger traffic is not high, but low cost combined with high top speed may be important to compete with automobiles.

Many light rail systems—even fairly old ones—have a combination of both on- and off-road sections. In some countries (especially in Europe), only the latter is described as light rail. In those places, trams running on mixed rights-of-way are not regarded as light rail, but considered distinctly as streetcars or trams. However, the requirement for saying that a rail line is "separated" can be quite low—sometimes just with concrete "buttons" to discourage automobile drivers from getting onto the tracks. Some systems such as Seattle's Link are truly mixed but closed to traffic, with light rail vehicles and traditional buses both operating along a common right-of-way.

Some systems, such as the AirTrain JFK in New York City, the DLR in London, and Kelana Jaya Line in Kuala Lumpur, have dispensed with the need for an operator. The Vancouver SkyTrain was an early adopter of driverless vehicles, while the Toronto Scarborough rapid transit operates the same trains as Vancouver, but uses drivers. In most discussions and comparisons, these specialized systems are generally not considered light rail but as light metro systems.

Energy efficiency for light rail may be 120 passenger miles per gallon of fuel (or equivalent), but variation is great, depending on circumstances.[23]

One line of light rail (requires 25' Right of Way) has a theoretical capacity of up to 8 times more than one 12' lane of freeway (not counting buses) during peak times. Roads have ultimate capacity limits that can be determined by traffic engineering. They usually experience a chaotic breakdown in flow and a dramatic drop in speed (a traffic jam) if they exceed about 2,000 vehicles per hour per lane (each car roughly two seconds behind another).[24] Since most people who drive to work or on business trips do so alone, studies show that the average car occupancy on many roads carrying commuters is only about 1.5 people per car during the high-demand rush hour periods of the day.[25] This combination of factors limits roads carrying only automobile commuters to a maximum observed capacity of about 3,000 passengers per hour per lane. The problem can be mitigated by introducing high-occupancy vehicle (HOV) lanes and ride-sharing programs, but in most cases the solution adopted has been to add more lanes to the roads.

By contrast, light rail vehicles can travel in multi-car trains carrying a theoretical ridership up to 20,000 passengers per hour in much narrower rights-of-way, not much more than two car lanes wide for a double track system.[26] They can often be run through existing city streets and parks, or placed in the medians of roads. If run in streets, trains are usually limited by city block lengths to about four 180-passenger vehicles (720 passengers). Operating on two-minute headways using traffic signal progression, a well-designed two-track system can handle up to 30 trains per hour per track, achieving peak rates of over 20,000 passengers per hour in each direction. More advanced systems with separate rights-of-way using moving block signalling can exceed 25,000 passengers per hour per track.[27]

Most light rail systems in the United States are limited by demand rather than capacity (by and large, most American LRT systems carry fewer than 4,000 persons per hour per direction), but Boston's and San Francisco's light rail lines carry 9,600 and 13,100 passengers per hour per track during rush hour.[28] Elsewhere in North America, the Calgary C-Train and Monterrey Metro have higher light rail ridership than Boston or San Francisco. Systems outside North America often have much higher passenger volumes. The Manila Light Rail Transit System is one of the highest capacity ones, having been upgraded in a series of expansions to handle 40,000 passengers per hour per direction, and having carried as many as 582,989 passengers in a single day on its Line 1.[29] It achieves this volume by running four-car trains with a capacity of up to 1,350 passengers each at a frequency of up to 30 trains per hour. However, the Manila light rail system has full grade separation and as a result has many of the operating characteristics of a metro system rather than a light rail system. A capacity of 1,350 passengers per train is more similar to heavy rail than light rail.

Bus rapid transit (BRT) is an alternative to LRT and many planning studies undertake a comparison of each mode when considering appropriate investments in transit corridor development. BRT systems can exhibit a more diverse range of design characteristics than LRT, depending on the demand and constraints that exist, and BRT using dedicated lanes can have a theoretical capacity of over 30,000 passengers per hour per direction (for example, the Guangzhou Bus Rapid Transit system operates up to 350 buses per hour per direction). For the effective operation of a bus or BRT system, buses must have priority at traffic lights and have their own dedicated lanes, especially as bus frequencies exceed 30 buses per hour per direction. The higher theoretical of BRT relates to the ability of buses to travel closer to each other than rail vehicles and their ability to overtake each other at designated locations allowing express services to bypass those that have stopped at stations. However, to achieve capacities this high, BRT station footprints need to be significantly larger than a typical LRT station. In terms of cost of operation, each bus vehicle requires a single driver, whereas a light rail train may have three to four cars of much larger capacity in one train under the control of one driver, or no driver at all in fully automated systems, increasing the labor costs of BRT systems compared to LRT systems. BRT systems are also usually less fuel-efficient as they use non-electrified vehicles.

The peak passenger capacity per lane per hour depends on which types of vehicles are allowed at the roads. Typically roadways have 1,900 passenger cars per lane per hour (pcplph).[30] If only cars are allowed, the capacity will be less and will not increase when the traffic volume increases.

When there is a bus driving on this route, the capacity of the lane will be higher and will increase when the traffic level increases. And because the capacity of a light rail system is higher than that of a bus, there will be even more capacity when there is a combination of cars and light rail. Table 3 shows an example of peak passenger capacity.

On the Karlsruhe Stadtbahn, trams sometimes share mainline tracks with heavy rail trains

The Trillium Line in Ottawa was built along a freight railway and is still occasionally used by freight traffic overnight.

Around Karlsruhe, Kassel, and Saarbrücken in Germany, dual-voltage light rail trains partly use mainline railroad tracks, sharing these tracks with heavy rail trains. In the Netherlands, this concept was first applied on the RijnGouweLijn. This allows commuters to ride directly into the city centre, rather than taking a mainline train only as far as a central station and then having change to a tram. In France, similar tram-trains are planned for Paris, Mulhouse, and Strasbourg; further projects exist. In some cases, tram-trains use previously abandoned or lightly used heavy rail lines in addition to or instead of still in use mainline tracks.

Some of the issues involved in such schemes are:

• compatibility of the safety systems
• power supply of the track in relation to the power used by the vehicles (frequently different voltages, rarely third rail vs overhead wires)
• width of the vehicles in relation to the position of the platforms
• height of the platforms

There is a history of what would now be considered light rail vehicles operating on heavy rail rapid transit tracks in the US, especially in the case of interurban streetcars. Notable examples are Lehigh Valley Transit trains running on the Philadelphia and Western Railroad high-speed third rail line (now the Norristown High Speed Line). Such arrangements are almost impossible now, due to the Federal Railroad Administration refusing (for crash safety reasons) to allow non-FRA compliant railcars (i.e., subway and light rail vehicles) to run on the same tracks at the same times as compliant railcars, which includes locomotives and standard railroad passenger and freight equipment. Notable exceptions in the US are the NJ Transit River Line from Camden to Trenton and Austin's Capital MetroRail, which have received exemptions to the provision that light rail operations occur only during daytime hours and Conrail freight service only at night, with several hours separating one operation from the other. The O-Train Trillium Line in Ottawa also has freight service at certain hours.

When electric streetcars were introduced in the late 19th century, conduit current collection was one of the first ways of supplying power, but it proved to be much more expensive, complicated, and trouble-prone than overhead wires. When electric street railways became ubiquitous, conduit power was used in those cities that did not permit overhead wires. In Europe, it was used in London, Paris, Berlin, Marseille, Budapest, and Prague. In the United States, it was used in parts of New York City and Washington, D.C.[43] Third rail technology was investigated for use on the Gold Coast of Australia for the G:link light rail,[44] though power from overhead lines was ultimately utilized for that system.

In the French city of Bordeaux, the tramway network is powered by a third rail in the city centre, where the tracks are not always segregated from pedestrians and cars.[45] The third rail (actually two closely spaced rails) is placed in the middle of the track and divided into eight-metre sections, each of which is powered only while it is completely covered by a tram. This minimises the risk of a person or animal coming into contact with a live rail. In outer areas, the trams switch to conventional overhead wires. The Bordeaux power system costs about three times as much as a conventional overhead wire system, and took 24 months to achieve acceptable levels of reliability, requiring replacement of all the main cables and power supplies.[46] Operating and maintenance costs of the innovative power system still remain high. However, despite numerous service outages, the system was a success with the public, gaining up to 190,000 passengers per day.

The BART railcar in the following chart is not generally considered to be a "light rail" vehicle (it is actually a heavy rail vehicle), and is only included for comparison purposes.

An important factor crucial to LRT is the train operator. Unlike rail rapid transit, which can travel unattended under automatic train operation (ATO), safe, high-quality LRT operation relies on a human operator as a key element. The reason that the operator is so important is because the train tracks often share the streets with automobiles, other vehicles, and pedestrians. If trains were fully automated on roads, nobody would be there to stop the train if a car pulled in front of it. Light rail trains are actually very sturdily built for passenger safety, and to reduce damage from impacts with cars.

The latest generation of LRVs has the advantage of partially or fully low-floor design, with the floor of the vehicles only 300 to 360 mm (11.8 to 14.2 in) above the top of the rail, a feature not found in either rapid rail transit vehicles or streetcars. This allows them to load passengers, including those in wheelchairs or strollers, directly from low-rise platforms that are little more than raised sidewalks. This satisfies requirements to provide access to disabled passengers without using expensive and delay-inducing wheelchair lifts, while also making boarding faster and easier for other passengers.

Overhead lines supply electricity to the vast majority of light rail systems. This avoids the danger of passengers stepping on an electrified third rail. The Docklands Light Railway uses an inverted third rail for its electrical power, which allows the electrified rail to be covered and the power drawn from the underside. Trams in Bordeaux, France, use a special third-rail configuration where the power is only switched on beneath the trams, making it safe on city streets. Several systems in Europe and a few recently opened systems in North America use diesel-powered trains.